This invention covers a method and apparatus of variable structure for controlling asynchronous motors. More particularly the invention teaches methods and apparatus for control of flux, torque, angular acceleration, angular velocity, and/or angular position of the rotor of an asynchronous machine supplied through a converter having switching elements. The method and apparatus are based upon variable structure control systems, and especially upon introducing sliding mode in the systems with discrete type control signal. Introduction of the sliding mode into the control system operation, for the asynchronous machine fed through a converter containing switching elements, is natural because the converter output voltage contains discontinuities that represent control actions to the induction machine. At the same time, such a concept makes possible incorporating a series of new and useful qualities, into the control system.
Prior art control systems have had a magnetic flux regulator, and a torque regulator, which changed the amplitude and frequency of the output voltage, or of the asynchronous motor's supply current, and consequently changed the amplitude and frequency of the stator current of the motor (see Abraham Ludvig, Vehrfahren zur Steurung des von ainer Asynchronmaschine abgegebenan Drehmoments/Licentia-Patent-Verwaltungs kl 21 224/02 HO2 p 7/42 No. 15 65 223). The magnetic flux controller has been a PI type controller, and it affected that component of the stator current which was orthogonal to the direction of magnetic flux of the asynchronous machine's rotor.
The control of the angular velocity of the rotor has been possible in an indirect way, by means of a PI type speed controller, which affected the input of PI type torque controller. The input signal to the speed controller was the difference between the set point value and the measured value obtained using transducer. When voltage inverters were used, the function generators were employed for computing the set point effective value of continuous output voltage of the invertor, the generators being adjusted according to the parameters of the asynchronous motor.
The known methods and systems for controlling, described in U.S. Pat. No. 3,824,437 to Felix Blaschke "Method for Controlling Asynchronous Machines", discuss the set point control of orthogonal projections of the current vectors in the system of coordinates oriented in the direction of rotor flux vector, which determine the modulus of the rotor flux vector torque, and velocity of rotation of the rotor. Magnetic flux transducers (Hall's generators) were used in that system for measuring the vector of the rotor flux, which requires employing a special motor with built-in transducers. Besides, this type of controllers of magnetic flux, torque, and velocity of rotation of the rotor, does not permit a direct regulation of angular position, velocity and acceleration of the rotor, and does not permit improving the dynamic processes of the control system.
The known methods of computing orthogonal projections of the rotor flux and velocity of rotation vectors (see Swiss patent Felix Blaschke: Einrichtung zur Istwertbildung fur die Regelung einer Drehstromasynchronmaschine" Holp 5/40 No.472146) are based on applying dynamic models of the stator and rotor circuit of asynchronous machine which operate on the basis of measured electric values that correspond to the voltage and currents of asynchronous machine.